Micromechanical structures are of growing interest for a number of applications in both research and industrial products. For example, it has been found that very small resonant structures are ideally suited to be used as sensors and force gauges. In one application, for example, it has been found that such structures may be used as chemical sensors, wherein a change in the resonant frequency of the structure, as molecules are absorbed into the surface of the structure, can be detected. A large surface area to mass ratio is important to the achievement for high sensitivity in such sensors. Further, it has also been found that high resonant devices can also yield valuable information about the physical properties of materials, and in particular about the sources of internal friction within such materials. Most of the small resonant structures that have previously been used for such purposes were fabricated using known photolithography and chemical etching techniques, and were fabricated with minimum dimensions of 1 micrometer or greater. This limited the resonant frequencies of such devices and limited their usefulness in certain applications.
The present invention is directed to the fabrication of nanometer-scale mechanical structures in single-crystal silicon or other suitable materials, to the operation of such nanostructures as sensors or force gauges, to the accurate measurement of the motion of such structures, and to the use of the motion of such structures to modulate light at high frequencies. The invention incorporates novel designs that result in nanostructures having decreased mass, to permit higher resonant frequencies than were previously available, while retaining a large surface area to improve the sensitivity of sensors using the structures. The invention also is directed to designs which control the optical properties of nanostructures to facilitate measurements of their motion. The foregoing advantages are obtained through the fabrication of suspended, or released, nanostructures that have spaced, well-defined, sub-wavelength dimensions. In the preferred form of the invention, the nanostructures have generally parallel features such as narrow bars or beams spaced apart by distances of less than the wavelength of the illuminating light used to measure the motion of the structures so as to form, in effect, an optical grating. Interferometric optical techniques can then be used to measure the motion of the structure, using illuminating light from a laser which has a wavelength about twice the distance between adjacent features.
In one form of the invention, a released nanostructure may be in the form of a rectangular mesh which may be used, for example, to explore the mechanical properties of silicon. By observing the shape of the spectrum produced by the vibration of the structure, much can be learned about the nature of energy dissipation in mechanical systems. For example, it has been observed that the attainable quality factors (Q factors) in resonators tend to decrease as the size of the resonator is decreased and the consequent resonant frequency is increased. The nature of the correlation between size and Q-factor can be studied by use of the fabrication techniques and the measurement apparatus of the present invention.
Briefly, the present invention is directed to a method of fabricating and operating resonant nanostructures which includes patterning and etching a silicon-on-insulator (SOI) wafer to fabricate a nanostructure which is suspended over the wafer substrate, and which is electrically isolated from the substrate. The fabrication method allows the structure to be integrated with electronic devices such as conventional CMOS structures on the same SOI substrate. The suspended nanostructure is actuated by applying a high frequency drive voltage between the nanostructure and the substrate to cause the nanostructure to vibrate. This motion can be measured by directing light of a predetermined wavelength onto the nanostructure and detecting variations in, or modulation of, the light reflected from the vibrating nanostructure. The detected light is then supplied to an analyzer to determine the frequency and amplitude of motion of the nanostructure. The nanostructure also can be used simply as a modulator for incident light, as by varying the high frequency drive voltage or by the application of a DC bias voltage, to change the frequency or amplitude of the motion of the nanostructure.
In the preferred form of the invention, the nanostructure is a low mass, high surface area mesh which is suspended by thin wires for motion with respect to the wafer substrate. The mesh preferably is generally rectangular in shape and is made up of regularly-spaced longitudinal features such as narrow beams or bars extending the length of the mesh, and lateral features such as narrow supporting beams or bars spanning its width and intersecting the longitudinal beams to produce multiple generally rectangular apertures through the mesh. The adjacent longitudinal beams are spaced apart by a distance less than the wavelength of incident light to produce a nanostructure in the form of a suboptical wavelength mesh. The incident light preferably is produced by a laser, so that when the nanostructure is vibrated at or near its resonant frequency, and the laser light is directed onto its top surface, the vibration produces changes in the intensity of light reflected from the mesh. The nanostructure is driven so that it vibrates at a low mechanical amplitude to provide a linear relationship between the amplitude of the reflected optical signal and the magnitude of the drive voltage. The suboptical wavelength mesh thus may act, in effect, as an optical grating. The mesh permits interferometric measurement of the motion of the structure while modulating incident light. The low mass and high surface area of the high frequency resonant mechanical structure make it useful not only for exploring the mechanical properties of the material from which the mesh is manufactured, but provides a device which is highly useful as a sensor and as a light modulator. By fabricating the nanostructure on the same substrate as an electronic device, control and measurement of the motion of the device are greatly facilitated.